This invention refers to a novel process for coloring aluminum or aluminum alloy pieces or objects whose surfaces have previously been subjected to anodic oxidation.
The term "aluminum" will hereinafter refer to the metal and its alloys, and "anodized aluminum" will refer to any aluminum or aluminum alloy piece or object whose surface has been subjected to an anodic oxidation.
Various processes for coloring aluminum are known, amongst which are those pertaining to the so-called "two-phase" technique, i.e., the formation of an aluminum oxide layer and then electrodeposition of colored metallic compounds.
Norwegian Pat. No. 69,930, which refers to the known two-phase technique, particularly contemplates the use of an acid electroplating bath through which an alternating current is passed and which contains, on the one hand, one or more salts of a group consisting of Fe, Co, Ni, Mn and/or Cr salts and, on the other hand, an amount less than 10 g/l of one or more soluble compounds of another group of salts consisting of As, Sb, Bi, Se, Te and/or Sn soluble compounds. With a bath of this type, various bronzed tones have been obtained on the surface of the aluminum articles when colored substances of metals such as Fe, Co, Ni or Mn were deposited on the pores of the oxide layer. To carry out the codeposition of the metals of the two groups, it has been necessary to maintain the As, Sb, Bi, Se and/or Sn content below a certain limit, in order to maintain a current density which can be used industrially.
This process presents various drawbacks. The current density, when the bath primarily contains nickel, should be as low as possible in order to avoid perforated discharges or a concentric spalling of the oxide layer. The duration of the treatment should be increased, to obtain adequate coloring, thus decreasing economical use of the process, on the one hand, and increasing the risk of disruptive discharges, on the other.
Along these same lines, it is known that British Pat. No. 1,022,927 claims a process for coloring aluminum wherein an alternating current is passed between an aluminum piece and a counterelectrode, particularly made of retort carbon or gas carbon, carborundum or aluminum. The bath contains a small amount of an Ni, Co, Cr, Cd, V, Au, Ag, Fe or Pb salt.
Since this latter process presented the same drawbacks as those indicated above, consideration was given to French Pat. No. 2,011,176 which claims the application of an asymmetric alternating voltage between an anodized aluminum article and a counterelectrode immersed in a bath containing salts capable of coloring the aluminum oxide layer.
On the other hand, to overcome the drawbacks of the prior art and to deposit tin on anodized aluminum, in order to obtain coloration, French Pat. No. 2,047,917 contemplates a process according to which an acid bath containing tin salts and a tin complex agent is used with the purpose of avoiding the oxidation of tin (II) to tin (IV). An alternating current, which is applied between the aluminum piece to be treated and and a tin or titanium counterelectrode, is passed through the bath. The pH of the bath is preferably in the range of from 0.8 to 1.5 and the current density can vary from 0.2 to 1.0 A/dm.sup.2, or it can even be above 1.0 A/dm.sup.2.
It is evident that only specific coloring of the tin can be obtained by applying this process, i.e., bronzed colors with greenish tints, which can reach black.
However, at present, the colorings required by the construction and architectural industries have warmer tones which cannot be obtained with the tin salt baths contemplated in French Pat. No. 2,047,917.